I was able to get a repo case set up that triggered quickly.
After printing the return from "variables, _ = popen.communicate()" statement at line 82 of setup_toolchain.py, the message written to stdout is "The input line is too long".
I think maybe in my build environment, every time a new batch file is called, (like when I hit "call cef_create_projects.bat") a new copy of the environment is added to the existing one - seems to happen a bunch - there are 4 copies of each environment variable at that point - and once it exceeds, 8191 bytes in length, that message appears.
This doesn't happen if I run the batch files directly from a command window and so that line is okay.